Concrete heat transmission U values

[QualityTime]

Hi:

I am comparing the ASHRAE U values for different thickness of concrete to

http://www.combustionresearch.com/Infra-Spec/infra-spec/uvalue.html.

and to

http://wiki.answers.com/Q/What_is_the_R_value_of_concrete

Can anyone explain the difference?

 

[walz]

The first one is giving the overall U-Values for the building materials. I think their units are metric (W/(m2.K)). And its saying that overall U-Value is reciprocal of sum of all the R-Values of wall/roof assembly.


The second one is showing the relation between R-Value and k-value. How one can obtain the R value from the k value.

Now its sometimes confusing. You have to understand each one

1. Thermal Conductivity - k value - Unit is W/(m.K)
2. Thermal Resistance - R Value - Unit is m2.k/W

it can be obtained from k-Value with the formula described in Wiki Answers

for a Wall(or roof) assembly made of different layers, first thing is to determine individual R-values of each layer. then compute total resistance Rt as

Rt = R1+R2+R3+....

Now U-Value is the reciprocal of Rt (U = 1/Rt)

U value is called overall overall heat transfer coefficient

its units are W/(m2.K)

 

[IRstuff]

Note also that in the US, some of these values are oten quoted with a set CUSU thickness of an inch, and will result in a value that embodies both SI and customary US units.

TTFN

FAQ731-376